Air is separated by cryogenic rectification conducted in air separation plants through the distillation of the air within distillation column systems that include higher and lower pressure columns. In such air separation plants, compressed and purified air is distilled in the higher pressure column to produce a nitrogen-rich vapor column overhead and a crude oxygen column bottoms also known as kettle liquid. A stream of the crude oxygen is further refined in the lower pressure column that operates at a lower pressure than the higher pressure column. This further refinement of the crude liquid oxygen within the lower pressure column produces an oxygen-rich liquid and a nitrogen-rich vapor column overhead. Oxygen-rich and nitrogen-rich liquid and vapor products can be produced in such air separation plants.
The higher and lower pressure columns are operatively associated with one another in a heat transfer relationship in which the oxygen-rich liquid produced in the lower pressure column is passed in indirect heat exchange with a stream of the nitrogen-rich vapor column overhead removed from the higher pressure column. This results in condensation of the nitrogen-rich vapor and partial vaporization of the oxygen-rich liquid to produce boilup and thus, initiation of the formation of an ascending vapor phase of the mixture to be distilled in the lower pressure column. The condensed nitrogen-rich vapor can be used in generating reflux for the distillation conducted in both the higher and lower pressure columns. In this regard, the reflux so generated can be fed exclusively to the higher pressure column. In such case, the lower pressure column can be refluxed with a nitrogen containing liquid stream withdrawn from the higher pressure column at a location thereof where such liquid stream has a higher concentration of oxygen than the column overhead of the higher pressure column that is condensed in the lower pressure column.
It is to be noted that the heat transfer relationship between the columns is made possible by the fact that the nitrogen-rich vapor is at a higher pressure within the higher pressure column than the oxygen-rich liquid within the lower pressure column. Since the nitrogen-rich vapor is at the higher pressure, it will be warmer than the oxygen-rich liquid and thereby will be able to be condensed by the oxygen-rich liquid. It is to be noted that since the lower pressure column operates at a lower pressure than the higher pressure column, the volatility spread between the oxygen and nitrogen will be greater than in the higher pressure column to also enable the further refinement of the crude liquid oxygen produced in the higher pressure column.
The indirect heat exchange between the oxygen-rich liquid and the nitrogen-rich vapor occurs in a heat exchanger known as a main heat exchanger or alternatively, as a condenser-reboiler. The heat exchanger can be of the down-flow type in which the oxygen-rich liquid flows in a downward direction to be partially vaporized. Such a down-flow heat exchanger can be of plate-fin, brazed aluminum design in which the passages containing fins are formed between parting sheets for the flow of the oxygen-rich liquid and the nitrogen-rich vapor. In another type of heat exchanger a set of tubes are provided that are enclosed by a shell. The oxygen-rich liquid is fed into the tubes and partially vaporizes to escape from the bottom of the tubes into the lower pressure column. The nitrogen-rich vapor is fed to the shell for contact with the tubes and thus, condensation through indirect heat exchange with the oxygen-rich liquid. As shown in U.S. Patent Appln. Ser. No. 2007/0028649, heat transfer can be enhanced by providing the inside of the tubes with an enhanced boiling surface and the outside of the tubes with fins. In U.S. Pat. No. 6,393,866 the placement of fins and enhanced boiling surfaces is reversed and the heat exchanger shown in this patent is operated by feeding the nitrogen-rich vapor into the tubes and the oxygen-rich liquid onto the outer surfaces of the tubes. In both plate-fin and shell and tube heat exchangers, the oxygen-rich liquid is collected in the lower pressure column with a liquid collector and then fed to the down-flow heat exchanger by means of a liquid distributor.
In another type of heat exchanger, known as a thermosiphon heat exchanger, the oxygen-rich liquid collects within a sump of the lower pressure column or a shell located outside of a bottom region of such column. The nitrogen-rich vapor is then fed to the heat exchanger which sits in liquid located in the sump. The liquid vaporizes within passages of such a heat exchanger and as the liquid vaporizes; its density decreases so that the liquid flows up the passages and is discharged with the vapor from the top of such a heat exchanger. Thermosiphon heat exchangers have similarly been based on both plate-fin and shell and tube designs.
With reference to U.S. Pat. No. 5,071,458, a hybrid arrangement of a down-flow heat exchanger and a thermosiphon type of heat exchanger is situated in the sump of a lower pressure column of an air separation plant. The thermosiphon heat exchangers are situated below the down-flow heat exchangers. Oxygen-rich liquid is collected in a distributor that is fed to the down-flow heat exchangers. Partial vaporization of the oxygen-rich liquid results in residual liquid being collected in the sump for partial vaporization of such sump liquid within the thermosiphon heat exchangers. Upon a cold shutdown of the air separation plant, liquid will tend to fall through mass transfer contacting elements overlying the bottom region of the lower pressure column to collect in the sump. Since the down-flow heat exchanger will not function when submerged, while thermosiphon heat exchangers will continue to function under such circumstances, the thermosiphon heat exchangers take over during a restart of the plant and the resulting heat exchange will cause the liquid level in the sump to drop to a lower level at which the down-flow heat exchangers will again be able to function.
As mentioned above, the ability of the nitrogen-rich vapor, produced in the higher pressure column, to be condensed by the oxygen-rich liquid, produced in the lower pressure column, is dependent upon the higher pressure in the higher pressure column over that obtained in the lower pressure column. An advantage of down-flow heat exchangers is that they can operate at a lower temperature difference between condensing and boiling streams than thermosiphon heat exchangers. Therefore, when the condensing reboiling function is taken advantage of solely through the use of down-flow heat exchangers associated with the lower pressure column, the higher pressure column can operate at a lower pressure than would otherwise be required with the use of thermosiphon heat exchangers. Since the pressure of the higher pressure column is a function of the degree to which air is compressed in the air separation plant, a reduction in the required pressure will lower electrical power costs incurred in compressing the air. However, in the hybrid arrangement discussed above, the thermosiphon heat exchanger will require warmer nitrogen than the down-flow heat exchanger in order to indirectly exchange heat with the oxygen-rich liquid. Therefore, the thermosiphon heat exchanger will act as a limit upon the degree to which operational pressure is able to be lowered in the higher pressure column.
As will be discussed, among other advantages of the invention that will be discussed in detail hereinafter, the present invention provides a hybrid main heat exchange system in which lower temperature differences are able to be obtained in the down-flow heat exchanger to in turn lower required pressures and operating costs of the air separation plant.